The present invention relates, in general, to the field of electrical machines
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
An electric machine of a type involved here includes a first part-winding, a second part-winding, a core having slots in which the first and the second part-windings are wound, and a switching device for connecting the first part-winding to the second part-winding. In the following description, the term “part-winding” relates to a winding unit which, optionally including a plurality of turns, can be connected to a winding head of the electric machine.
While star/delta startup is conventional in Europe as a starting measure for asynchronous motors for reducing the startup current, in America so-called part-winding circuits or part-winding startups are widespread. This type of startup has also been accepted as standard worldwide in refrigerator technology. With such part-winding circuits, only some of the slots have current flowing through them in the startup phase. The resultant increase in resistance and leakage in comparison with the operating circuit in which in general all of the slots have current flowing through them results in a reduction in the startup current, which naturally brings with it a corresponding reduction in the torque. The advantage of these circuits in comparison with star/delta startup consists in the fact that the motor is not isolated from the power supply system when changed over to the operating circuit, but the second part-winding is connected to the motor which is already “excited” by the first part-winding.
The disadvantage of a part-winding circuit is the asymmetrical field distribution in the startup phase, in which only the first part-winding is connected, since only some of the slots are magnetized. The resultant additional harmonic fields result in corresponding harmonic field moments, which, under certain circumstances, prevent complete runup of the motor. In this case, the motor remains “stuck” at a saddle speed during runup. This is particularly the case for the most widespread type of part-winding, in which the motor winding is split into two entirely identical part-windings.
Published European Pat. Appl. No. EP 0 238 940 B1 discloses a startup circuit which attempts to address the disadvantage of the asymmetrical field distribution in the startup phase. This circuit has, however, first and the second part-windings which are markedly different, and, during operation, currents of different levels flow into the feed lines to the respective part-winding. Accordingly, different switching devices are required for the two part-windings. This entails increased costs for the motor driving.
The object of the present invention consists in ensuring complete runup of an electrical machine with a startup circuit while keeping the costs as low as possible.
It would therefore be desirable and advantageous to provide an improved electric machine to obviate prior art shortcomings and to ensure complete runup with a startup circuit while keeping the costs as low as possible.